Gasoline hydrocarbon separation recovery process utilizing molecular sieves



Jan. 5, 1960 G. F. FELDBAUER, JR., ETAL 2,920,038

GASOLINE HYDROCARBON SEPARATION RECOVERY PROCESS UTILIZING MOLECULAR SIEVES Filed Dec. 5. 1956 O: as s I 5; 1 1 8 2' n a 1' .3 J, 5 n 3 f f E r 8 I g a T g 1 (in g I O. 3 9 g3 :25

O 7 George E Feldbuuer, Jr. Charles E. Johnig Inventors Norman J. Weinstein WW M Attorney .property may be mentionedchabasites.

GASOLINE HYDROCARBON SEPARATION RE- COVERY PROCESS I SIEVES Application December 5, 1956, Serial No. 626,358

9 Claims. (Cl. 208-310) UTILIZING MOLECULAR The present invention relates to a process for separating and segregating straight chained hydrocarbons from mixtures thereof with branched chain or cyclic hydrocarbons. More particularly, the present invention relates to the separation of relatively straight chain hydrocarbons from branched chain isomers employing a class of natural or synthetic adsorbents termed, because of their crystalline patterns, molecular sieves. Still more particularly, the present invention relates to an improved desorption process whereby the hydrocarbon adsorbed on the sieve is recovered in a manner resulting in higher recovery in a higher state of purity and adsorbent'efliciency than hitherto found possible. I

, It has been known for some time that certain zeolites, both naturally-occurring and synthetic, have the propertyof separating straight chainfrom branched chain hydrocarbon isomers, as well as from cyclic and aromatic compounds. .These zeolites have innumerable cavities with. entrance pores of uniform size, and only molecules small enough to-enter the pores can be adsorbed. The pores may vary in diameter from 3 or 4-Angstroms to l5 or more, but it is a property of these zeolites that any particular sieve has pores ofa substantial uniform size.

Among the natural zeolites having this sieve A synthetic zeolite with molecular sieve properties is described in U.S. 2,442,191. Zeolites vary somewhat in composition, but generally contain the elements of silicon, aluminum and oxygen as well as an alkali and/ or an alkaline-earth zeolites.

element, e.g. sodium and/or calcium. The naturally occurring zeolite analcite, for instance, has the empirical formula NaAlSi O .I-I O. Barrier (U.S. 2,306,610) taught that all or part of the sodium is replaceable by calcium to yield, on dehydration, a molecular sieve having the formula (Ca, Na Al Si O .2I-I O. Black (U.S. 2,522,426) describes, a synthetic molecular sieve zeolite having the formula 4CaO.Al O .4SiO A large number sieve activity, i.e. the ability, to adsorb a straight chain the olefinic desorbent.

hydrocarbon and exclude thexbranch chain isomers due,

to differences in molecular size, are described in an article Molecular Sieve Action of Solids appearing in, Quarterly Reviews, vol. III, pages 293-320 (1949), published by the Chemical Society (London).

The segregation or removal of branched chain or depend upon the specific structure of the various possible hydrocarbons utilizable as reactants in the preparation or manufacture of the final product. Thus in the preparation 'o'f high-octane fuels the presence of paraf-- &

-causes them to deteriorate.

2,920,038 Patented Jan. 5, 1960 "ice separations, i.e. of straight chain from branched chain. and cyclic hydrocarbons by molecular sieves, and though excellent and selective separations have been realized, a

' serious problem has arisen when it was attempted to" installations, it is necessary to employ a cyclic operation,

i.e. an adsorption step followed by desorption and regeneration of the sieve, and thereafter another adsorption step. .It has been found that regeneration by the conven .v tional methods of heating, evacuation, steaming and thelike result in a marked decline in the adsorptive capacity of the sieves. For instance, in a process'wherein a virgin naphtha fraction was treated with a synthetic molecular sieve having a pore diameter of 5 Angstroms to separate straight from branched chain and cyclic constituents, and the sieves desorbed and regenerated between cycles by steam stripping followed by nitrogen flushing, the sieves decreased in capacity to 59% of fresh capacity after only three cycles. Similarly, the'elfect of continued treatment of the sieves with steam at the high stripping temperature Substantially. improved, desorption has been realized when there is employed as a desorbent and regenerating means a low boiling, preferably gaseous olefin, which is itself in turn readily desorbed, at the end of the desorbing cycle, by the higher molecular weight normal paraffins.-

An important problem associated with an economic olefin desorption process is the recovery and re-use of- Thus, in the cyclic operation briefiyloutlined above, the eflluent from a molecular sieve adsorption zone contains, during the n-parafl'ln' adsorption part of the cycle, a mixture of n-paraffin-free hydrocarbons and olefin. This vaporous' mixture mustand stripping hydrocarbons adsorbed with uniform pores and cavities of certain natural or synthetic zeolites, commonly called molecular sieves, in a manner more economic than hitherto found possible.

It is also a purpose of the present invention to provide desorption means which prolong substantially the life of the molecular sieve adsorbent.

It is a still further object of the present invention to desorb hydrocarbons from molecular sieveswithout employing unduly high temperatures, and in addition achieve the advantages of a substantially isothermal operation. Isothermal operation is possible because the heat of adsorption of the n-paraflins and low boiling olefins are approximately equal. 7

A still further object of the present invention is :to -provide a means of adsorbing straight chain paraflini'c compounds and desorbing them with olefinic compounds in a manner whereby compression of vapor in recycle streams are substantially avoided and large economic savings and advantages are realized.

It is a still further object of the present invention to provide a means of desorbing straight chain aliphatic hydrocarbons from molecular sieves whereby improved hydrocarbon feed stream for petroleum refinery processes are obtained.

Other and further objects of the present invention willappear in the following more detailed description and claims.

In accordance with the present invention, fractionation of the desorbate stream is avoided by the use of a particular adsorption technique. A high boiling hydrocarbon, preferably. a cut that is normally put into the gasoline pool, is employed as a wash oil to scrub the isoparaifins and other non-normal constituents from the desorbate streams. The remaining olefin is thenpure enough for direct reuse in desorption. In this way, the cost for separating the olefin. from the effluent is substantially reduced, and, by use of a gasoline component for the wash\ oil,.recovery problems and the necessity for recycle are avoided.

The relationship between the olefin desorbent, the paraffin desorbed, andthe preferred desorbing temperature are shown in the table below:

In one embodiment of the invention, a mixed branchedchain andcyclic/ straight chain material, such as virgin naphtha, boiling in the (I -200 F. range may be passeds through a bed of molecular. sieves, having pores of about. A. until just before normal parafifins appear in the chinent. Thereupon, without changing the temperature of the molecular, sieve bed, anolefin-containing gas stream, preferably containing propylene, is passed through the bed until the parafiin has. been substantially displaced. Thereafter, the cycle is repeated.

The process of the present invention may be understood more clearly when read in conjunction with the drawing.

The figure is a flow plan representing a simple adsorption-desorption cycle including the heavy wash oil separation technique.

Turning now to the figure, there is shownv an adsorp. tioncolumn 16, containing the molecular sieve adsorbentL The size of the pore diameterdepends upon the molecular size of the material to be separated. It must be large enough to adsorb the straight chain but not large enough to adsorb the branched chain isomers. The adsorptive capacity and pore size of the sieve, and the structure of the hydrocarbon are related in the following manner:

fins to about 95 when this is reduced to 810%.

Adsorbed on Adsorbed on 5 A. Not adsorbed on Adsorbed on 4 A. and 5 A. but not 4 A. 4 A. or 5 A. 13 A.

(1) Ethane. (1) Propane and (1) Iso-parafiins. (1) All hydro: y

hi her nparcarbons a s. within gasoline boiling range. (2) Ethyl- (2) Butene and (2) Aromatics. (2) Aromatics one. higher n-olestrongly fins. adsorbed. (3) Propyl- (3) All cyclics (3) Diolcfins ene. with 4 or more strongly atoms in ring. adsorbed.

In general, when it is desired to increasethe octane.

A motor fuel prepared by the hydroforming of a hydrocarbon fraction, and boiling in the range of C '-200 'F., containing substantial amounts of naphthenes and aromatics as well as a minor proportion of normal paraflins is employed as feed in one embodiment of the invention. Because of the presence of the normal paraffins, the octane number of the hydroformate is relatively low, and may;

vary from when the feed-contains about 15-20% paraf- Agfeed of this type, which may contain small amounts of moisture or sulfur compounds, is introduced into thedesorption system through line 2, and may be passed through preliminary feed purification zones. 8'01". 10. The feed may be preheated to a temperature of 200 to 400 F. Thisprehe'at is preferably accomplished byheat exchange with efiiuent fromthe adsorption step. Zones 8 and 10 contain a molecular sieve material such as 5 A., 4 A. or less. It has been found that the capacityof sieves to ad- I sorb hydrocarbons is greatly reduced if water is present,

even in small quantities, since it is more strongly adsorbed than most hydrocarbons. Certain sulfur compounds are also selectively adsorbed and diflicult to desorb. Since most hydrocarbon streams available in a refinery contain small amounts of these impurities, the continued use of the sieves insuch separation operations may necessitate periodic interruptions to desorb the contaminants and restore adsorbent capacity. With-the use of the 4 A. type sieve, contaminants are removed but the hydrocarbons are not adsorbed. Since the capacity of the sieves for water is high, zones 8 and 10' prevent water from getting into tower 16. In the drawing, two such water adsorptionsystems are employed in drying and regeneration cycles so as to make the hydrocarbon separationcontinuous.- Each of the systems in turn may comprise two alternate Zones,

the feed being switched from 8 to 1t] when zone 8 requires regeneration.

The latter is accomplished by sweeping out the water with hot gases-such-as air. Itis understood,

however, that a clean, dry feed or onethat is substantially sulfur-free,- may not require thispurification treatment.

The hydrocarbon feed is now passed, preferably in the vapor phase at a temperature of about 200 to 400 F.

into adsorption tower 16. The'adsorbent, which may be any natural. or synthetic zeolite of the molecular sieve type heretofore described, may be arranged in trays, or packed on supports or be unsupported. Reaction conditions within adsorber are flow rates of 0.1-5 v./v./hr.,

temperatures of 175 -350 R, and pressures of 5 to about p.s.i.g. High pressures cause non-selective adsorption in some cases. for subseqent propylene recyclecompression, the adsorption step is preferably carried out atv a higher pressure than the desorption step.

The substantially straight chain paraffin-free naphtha In orderto avoid or minimize the need I is withdrawn from tower 16 and passed via line 17 and index, gravity or spectrographic analysis ofthe effiuent, the flow-of hydrocarbon feed through line 2 is halted and the desorption cycle begins. An olefin-containing gas, preferably one comprising a substantial proportion of propylene, and preheated to 200 to 400 F. is passed through linel, dried if desired in purifiers 7 or 9 containing thesame or similar sieve type as in zones Sand 10, and passed. into tower 16. Cracked refinery gases,- containing besides propylene, minor. amounts of. ethane and propane and butylene may be used for this purpose.

Without changing the temperature of the tower 16 apsielz uzdQWIblllflgfifiS. rsplasc tt P tamnkade.

The initial first cycle adsorbate is as determined by conventional means such as refractivesorbed on the sieves withthe olefins. The pressure, however, is decreased so that, during the desorption cycle, it is in the range of 0 to 20 p.s.i.g. This alternate high pressure adsorption and low pressure desorption eliminates or minimizes the need for recycle olefin compression. A good method for depressuring after the adsorption step is to vent the vapors from vessel 16 to drum 42 maintained just below the desorption'pressure, where the vapors are quenched with cold feed.

The paraflinic constituents of the sieve are displaced by the light olefinic desorbing gas. The latter is of lower molecular weight than the naphtha,'since the zeolites have a substantially greater aflinity for olefins than for paraflins of the same number of carbon atoms. However, some of the olefins may also break through, and separation of olefin from n-paraffin is therefore required. The desorbed n-paraflins are withdrawn through line 18, cooled in cooler 26, and passed to accumulator-separator 20. The olefinic desorbent gases may be withdrawn through line 24 and may be recycled or employed as desired. The n-paraflin produced from line 22 may be isomerized, aromatized or reformed, all in a manner known per se. At the end of the desorption cycle, adsorption is resumed.

The vessel 16 is repressured to the adsorption pressure of to 100 p.s.i.g. This may be readily accomplished by diverting part of the vaporized feed. During this pressuring step, an incremental feed stream may be added to the feed just upstream of the drier to minimize fluctuation in the feed going to vessel 16.

The olefins are now in turn desorbed by the n-paraflins. Effluent from vessel 16 now withdrawn through line 17 comprises a mixture of n-paraffin-free hydrocarbons plus propylene. This mixture must beresolved into its components. In accordance with one embodiment of the present invention, it is passed through a partial watercooled condenser 25 and the partially condensed product passed to separator 28. A portion of the heavier isoparaffins may be separated as a liquid at this stage and passed via line 2? to stripper 31. Here, reboiled n-paraffin-free hydrocarbons are used to strip out any light olefins, and these are returned via lines 37 and 27 to vessel 28. The product withdrawn through line 33 is high octane isoparaflins and/or cyclic hydrocarbons, free of propylene desorbent.

Returning now to the main stream, a mixture of uncondensed isoparaflins, aromatics, naphthenes and the like, along with the olefin desorbent is passed upwardly through line 30 into absorber 32. Countercurrently, a liquid stream of wash oil, preferably a constituent of a gasoline pool, is admitted through line 34 and passed downwardly. The scrubbing medium may be a heavy hydroformate or other heavy stream such as alkylate, or a catalytic naphtha cut. A preferred scrubbing medium is catalytic reformate boiling about 270 F. This material is composed almost entirely of xylene and heavier aromatics. Heavy naphtha from catalytic cracking can also be used.

The unabsorbed propylene is withdrawn through line 36 and may, without substantial repressuring, be recycled to a second adsorption column 16' which is being desorbed. The operating pressures are chosen to eliminate the propylene recycle compressor normally required. The wash oil containing the n-paraffin-free product is withdrawn downwardly through line 38 and may be passed directly to the gasoline pool if the scrubbing medium is itself a desired gasoline constituent. On the other hand, if a heavier composition such as gas oil is employed as a scrubbing medium, the isoparaflins, etc., are separated from the mentsruum in a stripping zone and the heavy hydrocarbon recycled.

. As has been pointed out, the need for compression of recycle gases is further minimized or substantially avoided by carrying out the adsorption step at a significantly 6' higher pressure than the desorption step within critical pressure ranges. Above about p.s.i.g., adsorption becomes non-selective, and desorption at less than atmospheric pressure results inv uneconomical operation.

' The diflerence in pressure between the adsorption and the desorption step is determined by the pressure drop through the desorbent-isoparaflin product separation system described.

A convenient and preferred means of depressuring and repressuring the molecular sieve zone is as follows:

After the adsorption step has been completed, valve 50 is opened and the vapors contained within adsorption column 16 are allowed to flow through lines 40 and 44 into quench vessel 42 maintained at a temperature corresponding to a pressure just below the preferred desorption pressure. In the case of a C /ZOO" F. light virgin naphtha, the temperature would be F. at a pressure of 4 p.s.i.g. This temperature is maintained by mixing cold feed with the hot vapors removed from the adsorption column. The hot vapors from the adsorption column and the cold liquid feed may be contacted in any conventional manner such as jets, packed columns, disc-and-doughnut columns, and the like.

After the desorption step, column 16 is repressured with a portion of the feed'which has been brought to the adsorption pressure in the liquid state by means of a sorbing agents as well aspropylene, particularly, for

higher molecular weight paraffins. The separations may be employed for substantially any feed containing straight' chain compounds such as n-parafiins, the sieve pore diameter being chosen in accordance with the molecular size. Though a fixed-bed operation has been described, the separation cycles may also be carried out by means of the so-called fluidized solids technique in fluidized beds or by moving bed techniques. The process of the present invention is particularly adapted to be employed in association with various means for upgrading virgin naphthas to form high octane motor fuels. As'pointed out, the process is advantageously employed in connection with a fluid or fixed bed hydroforming operation wherein naphthas are treated at elevated temperatures and pressures in the presence of a catalyst such as platinum with hydrogen under conditions to convert a substantial portion of the hydrocarbons present to aromatics. The resulting hydroformate is then enhanced in octane value by removal of n-parafiins by the-sieves. The adsorbate may then be recovered in accordance with the process of the present invention and recycled to the hydroformer for further conversion, or isomerized.

Furthermore, the heavy hydrocarbons may be introduced through line 37 prior to passage through product condenser separator 28. This has the advantage of eliminating tower 32. The total product is now passed downwardly through stripper 31, and the heavy hydrocarbon n-paraflin-free product withdrawn through line 33.

is to reverse the flow stream, i.e. feeding into the vessel during the adsorption cycle at the opposite end from that where the desorbent is fed during the previous cycle.

This technique, in combination with stopping short of breakthrough, provides a buffer zone of sieves, and prevents n-paraffins from mixing with the n-paraflin-free product.

A particular desirable manner of operating vessel 16 I In one modification; molecular sieves are used in the separation of n-paraflins from 'a catalyticallyreformed light virgin naphtha. Inone case a 93 octane number reformate was treated with 5 A. sieve to give the following results:

Yield, vol; percent on reformate 86.5 Res." Octane No. 101

In'addition, a 13.5% yield of n-paraffins was recovered which is available for reprocessing. If the virgin naphtha'had been reformed to 101 RON (road octane number), the yield on virgin naphtha would have been 58%. First reforming, then separating the n-paraffins with a 5 A. sieve gives a yield of 63%, assuming discard of the n-paraffins. The total combination yield of 101 RON gasoline may be increased to 69% by reforming orsisomerizing the n-paraffins. Thus, in this case, catalytic reforming to 93 RON plus a 5 A. mo ecular separation to improve the reformate octane rating, leads to 5 to 10% higher yields than severe reforming to the same octane level.

The high octane product resulting from sieve separations contain to 2% n-parafiins depending upon operating procedure. The lower parafiin contentsare obtained by stopping adsorption short of massive breakthrough. In order to keep the n-paraflin content to values below 1%, it is necessary that the feed and desorbent streams enter opposite ends of the bed. This overcomes the possibility of immediate n-paraffin breakthrough due to buildup in a zone where the desorbent concentration is low. It is also necessary to use a minimum of one liquid volume'of desorbent to replace one liquid volume ofh-parafiins. Use of lessdesorbent will not suflicient 1y, remove the adsorbed n-paraffins and will cause gradual increase in n-parafiin content of the high octane product stream, for a given set of operating conditions.

Adsorption at higher pressure than desorption, and the use of one liquid volume of desorbent to one liquid volume of n-paraffin, contributes to elimination of the need for recycle compression. The molecular sieve bed provides desorbent capacity and the pressure difference provides the driving force necessary to recycle the desorbent stream. Where equilibrium considerations re' quire the use of greater amounts of desorbent, partial recycle compression is needed. Only the desorbent which breaks through with the n-parafiins during desorption need be compressed for recycle; The rest of the stream is stored in the-molecular sieve bed. The amount of propylene needed for desorbing C to C n-paraffins varies from one to three liquid-volumes per liquid volume of n-paraflins. The amount of butylene needed is somewhat less. The greater the quantity of desorbent used, the greater will be the useful capacity of the sieve.

The quantity of high octane product taken overhead in the desorbent stream can be set by the adsorption equipment design. It is necessary and practical to reduce this potential loss to negligible amounts. The quantity lost may range from 0.05 to 2% on feed. The lower quantities may be obtained by adequate heavy wash oil coupled with sufiicientcontacting stages. The use of agasoline component Stream for absorption readily provides suflicient Wash oil and eliminates the need for wash oil recovery. This combination allows almost 100% recovery of the high octane product.

'Som'e. cyclic and branched chain hydrocarbons may be adsorbed in the molecular sieve bed at elevated pressures. The mechanism involved may be adsorption on the molecular sieve binder, adsorption on the crystal surface, or adsorption inthe crystal cavity due to momentary'or permanent expansion of the pore openings. This non-selective adsorption is not important up to pressures of about-3 to 4 atmospheres, and the-normal paraffin The use of low pressures will-lreep-:

this-loss to 0 to 2% on feed. Very low lossescan be: obtained by depressuring the adsorption vessels by direct. quench with cold feed. The cyclic and branched chain hydrocarbons which are adsorbed will desorb first during depressuringand be recycled with the feed.

Three or more reactor vessels, identical to vessel 16, are used to allow a continuous processing scheme. times, one or more vessels are being used for adsorption, one or more are being used for desorption, and one or more are available for pressuring or depressuring. Additional spare vessels may be used where periodic regeneration of the molecular sieve is desirable.

What is claimed is:

1. In a process for the separation of straight chain.

hydrocarbons from a feed mixture boiling in the gasoline boiling range and containing non-normal paraflin constituents by contacting said mixture. with molecular sieves in an adsorption zone whereby said straight chain hydrocarbons are selectively adsorbed and said non-normal parafiin constituents unadsorbed, and wherein said molecular sieves are desorbed with an olefin comprising gas and said olefin comprising gas is itself adsorbed, and

wherein said olefin containing gas is in turn desorbed by straight chain hydrocarbons present in said hydrocarbon mixture again fed to said adsorption zone, and a stream comprising said olefins and non-straight chain hydrocarbons unabsorbed from said feed mixture is withdrawn.

from said zone, the improvement which comprises scrubbing said stream with a hydrocarbon fraction boiling in the gasoline range, .and recycling unadsoroed olefins to said zone.

2. The process of claim 1 wherein said scrubbing medium is a catalytic reformate boiling above about 270 F.

3. The process of claim 1 wherein said olefin is a propylene comprising gaseous fraction.

4. The process of claim 1 wherein said scrubbing medium and adsorbed hydrocarbons are passed directly to a gasoline pool.

5. An improved process for separating straight chain parafiinic hydrocarbons from mixtures with non-straight chain hydrocarbons boiling in the gasoline boiling range Which comprises passing a vaporized stream of Said mixture into a molecular sieve adsorption zone, said molecular sieves having a pore diameter of about 5 Angstrom units, maintaining a temperature in said Zone of from about 175 to about 400 F. in said zone, maintaining a pressure of from about 5 to about p.s.ig. in said zone, withdrawing unads'orbed non-straight chain hydrocarbons from said zone while adsorbing straight chain hydrocarbons, thereafter decreasing the pressure in said zone to desorption pressures, passing a propylene comprising gas into said zone whereby normal paraffins are desorbed and said propylene adsorbed, maintaining a pressure of from about 5 to 20 p.s.ig. in said zone during said desorption step, said pressure being lower than during the adsorption step, withdrawing said normal desorbed hydrocarbons, thereafter repressuring said molecular sieve containing zone to adsorption pressures, passing said first named vaporized mixture into said zone, whereby said propylene is desorbed, passing said propylene and unadsorbed non-normal hydrocarbons into a scrubbing zone, scrubbing said material with a hydrocarbon fraction boiling in the gasoline range, and recycling unadsorbed propylene to said molecular sievecontacting zone.

6. The process of clairnS wherein said zone is depressured immediately following said adsorption stage by passing at least a portion of the vapors from said zone to a quenching zone, quenching said vapors with cold feed. at a temperature corresponding to a pressure below thedesired desorption pressure, and recycling said quenched product to said molecular sieve zone until desorption'pressure obtains within said zone.

7. The process of 'clai t fiwherein" said"'-molecular At all 9 sieve zone is repressured after the desorption cycle to adsorption pressures by passing a portion of the liquid feed at adsorption pressures to said zone. a

8. The process of claim 5 wherein said vaporized mixture is a naphtha fraction boiling in the range of C to 200 F. and said adsorption-desorption temperature is in the range of 200-400 F.

9. The process of claim 5 wherein said scrubbing medium is a catalytic reformate rich in aromatics and boiling above about 270 F.

References Cited in the file of this patent UNITED STATES PATENTS Black Sept. 12, 1950 Hirschler Nov. 14, 1950 Vesterdal et a1 Feb. 26, 1952 Eagle et a1 Feb. 17, 1953 Johnston ct a1 May 15, 1956 Richmond et a1 Dec. 31, 1957 Christensen et a1 Dec. 31, 1957 Ballard et a1 Dec. 31, 1957 

1. IN A PROCESS FOR THE SEPARATING OF STRAIGHT CHAIN HYDROCARBONS FROM A FEED MIXTURE BOILING IN THE GASOLINE BOILING RANGE AND CONTAINING NON-NORMAL PARAFFIN CONSTITUENTS BY CONTACTING SAID MIXTURE WITH MOLECULAR SIEVES IN AN ADSORPTION ZONE WHEREBY SAID STRAIGHT CHAIN HYDROCARBONS ARE SELECTIVELY ADSORBED AND SAID NON-NORMAL PARAFFIN CONSTITUENTS UNADSORBED, AND WHEREIN SAID MOLECULAR SIEVES ARE DESORBED WITH AN OLEFIN COMPRISING GAS AND SAID OLEFIN COMPRISING GAS IF ITSELF ADSORBED, AND WHEREIN SAID OLEFIN CONTAINING GAS IS IN TURN DESORBED BY STRAIGHT CHAIN HYDROCARBONS PRESENT IN SAID HYDROCARBON MIXTURE AGAIN FED TO SAID ADSORPTION ZONE, ANS A STREAM COMPRISING SAIL OLEFINS AND NON-STRAIGHT CHAIN HYDROCARBONS UNABSORBED FROM SAID FEED MIXTURE IS WITHDRAWN FROM SAID ZONE, THE IMPROVEMENT WHICH COMPRISES SCRUB- 